Dark earth gun with od gear?
 

Would a dark earth/tan gun look good with OD gear? I'm planning to get the elite force tar 21 as my next gun, and I don't like how it looks in black

don't worry about it, it will work

Dark earth is a great look for a gun, but dark earth and OD meld well anyways.

And would the tavor work in CQB?

There's nothing about the tavor design that's inherently 'bad' for cqb, whether it works for you or not comes down to your personal skills and preferences.
If at all possible, before buying it just take a minute to pick it up, see how it feels. Can you reach all the controls easily? Do the sights align naturally when you shoulder it? Does the weight feel well balanced to you? Are there any jutting parts that'll annoy you to death while carrying it slinged on your back?
These sorts of things will affect your experience with the gun far more than whether its colour matches your gear or not.